Polymerisation reactors have been used for a variety of polymerisation processes. Reactors may be suitable for batch processes or continuous processes. The present invention is concerned with the latter type. Most currently available continuous polymerisation reactors are dynamic systems having some moving parts which effect the mixing of reagents and, where necessary catalyst, and which force a reaction mixture through the reactor. Dynamic reactors require a fair amount of maintenance and are subject to potential breakdown of some of the moving parts. Static continuous reactors, where no moving parts are used to force the reaction mixture through the reactor, are also known. Adequate mixing in these reactors is mostly achieved through adapted internal geometry and/or the presence of internal parts, eg baffles, in the construction of the reactor.
In existing static continuous reactors the residence time of the reagents may be quite extended, especially where efficient distribution of a catalyst and heat transfer are critical. There is still a need to provide a polymerisation process which will allow the formation of polymers in an improved manner by using an efficient static reactor.
EP-A-522 776 discloses a process for making liquid polymers by condensing monomers and/or oligomers in a polymerisation reactor. The process involves mixing monomers and/or oligomers with an appropriate amount of catalyst, mixing the resultant reaction mixture with a pressurised gas to cause it to reach a foam like consistency, feeding the foaming mixture through an inlet into a reaction chamber having a porous wall and causing the monomers and/or oligomers to polymerise. In one embodiment the inlet is an atomising device, which in conjunction with the use of additional pressurised gas and the use of a narrow reaction chamber, encourages the formation of foam. Foam creates a large air-liquid interface, encouraging the use of the process for condensation polymerisation reactions. The reactor, however, needs to be specially built for the process.
EP 0657209 discloses a process for the preparation of liquid polymers via a condensation reaction in which monomers and/or oligomers are mixed with a catalyst and dispersed by means of an atomising device to form small droplets in a reaction chamber and are caused to polymerise in the dispersed state. In this case the amount of catalyst used is added as a predetermined constant amount, which was typically about 0.001 to 5% by weight of the catalyst based on the weight of the organosilicon compounds in the premix composition. Compressed air, used to atomise the premix, was fed into and extracted from the system at the same rate.
Attempts to make liquid polymers such as organosilicon polymers on a continuous basis having a defined final viscosity range are disclosed for example in U.S. Pat. No. 4,250,290 and GB 1,174,219; U.S. Pat. No. 4,250,290 discloses a process for the continuous manufacture of diorganopolysiloxanes wherein a catalyst is added to cyclic polysiloxane monomers and the resulting mixture is passed into and through a static mixer by means of plug type flow. The polymer viscosity is controlled in a two fold manner, firstly by adding to the initial part of the mixer a chainstopping agent and secondly by inactivating the catalyst when polymerisation is completed. GB 1,174,219 discloses a process wherein the starting materials were first mixed in a mixing chamber. The resulting mixture was forced into and through a reaction chamber into a discharge member. The reaction chamber was fitted with at least one degassification zone along its length whereby gases may be removed from the reacting mixture. The viscosity of the final reaction product is controlled by the speed at which the mixture passes through the reaction chamber.
Generally, the prior art overcomes the problem of production of a polymer reaction product with an inappropriate viscosity by taking periodic viscosity measurements and using an additional mixing step whereby the reaction product is mixed with polymer of a higher or lower viscosity with a view to obtaining a polymer of predetermined required viscosity. Mixing steps of this kind result in a reaction product with a wide molecular weight distribution, a feature known, in some instances, to adversely affect the mechanical properties of the resulting polymer. Furthermore the use of an additional step leads to a longer and therefore typically more expensive process.
DE 3914912 describes a system for obtaining a high molecular weight polymer having a required viscosity whereby the viscosity of the polymer is measured as it leaves a reaction chamber and any polymer obtained which is found not to be in the range required is automatically discharged and recirculated back into the reaction chamber once the catalyst contained in the discharged polymer has been neutralised or deactivated. However, this means there is a significant period of dead time between determination of the viscosity and recirculation of sample and therefore a potentially major portion of reaction product with an incorrect viscosity will be released from the reaction chamber prior to commencement of recirculation and any change of reaction conditions or the like.